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__BUILTIN_CONSTANT_P(3)    Library Functions Manual    __BUILTIN_CONSTANT_P(3)

NAME
     __builtin_constant_p - GNU extension to determine compile time constants

SYNOPSIS
     int
     __builtin_constant_p(value);

DESCRIPTION
     The __builtin_constant_p() is a GNU extension for determining whether a
     value is known to be constant at compile time.  The function is closely
     related to the concept of "constant folding" used by modern optimizing
     compilers.

     If the value is known to be a compile-time constant, a value 1 is
     returned.  If __builtin_constant_p() returns 0, the value is not a
     compile-time constant in the sense that gcc(1) was unable to determine
     whether the value is constant or not.

EXAMPLES
     A typical example of the use of __builtin_constant_p() involves a
     situation where it may be desirable to fold the computation if it
     involves a constant, but a function call is needed otherwise.  For
     instance, bswap16(3) is defined in NetBSD as:

           #define bswap16(x) \
                   (__builtin_constant_p((x)) ? \
                    __byte_swap_u16_constant(x) : __BYTE_SWAP_U16_VARIABLE(x))

SEE ALSO
     gcc(1), __builtin_object_size(3), __builtin_return_address(3)

CAVEATS
     This is a non-standard, compiler-specific extension.

NetBSD 8.0                     December 19, 2010                    NetBSD 8.0